The CBC/BBS trend shows Cuckoo abundance to have
been in decline since the early 1980s. The species was moved
in 2002 from the green to the amber list, and in the latest
review met red-list criteria. The sensitivity of CBC to change
in this species may have been relatively low, mainly because
Cuckoo territories were typically larger than census plots (Marchant
et al. 1990). BBS shows a continuing strong
decline in England and Wales, but not in Scotland. Cuckoo numbers
may have fallen because the populations of some key host species,
such as Dunnock and Meadow
Pipit, have declined (Brooke
& Davies 1987). Decreases among certain British
moths may have reduced food supplies for returning adults, and
the species may also be suffering difficulties on migration
or in winter (Glue 2006).
Strong variation in Cuckoo population trends between habitats
may reflect regional differences in the main hosts and differing
trends in Cuckoo breeding success among those host species:
Cuckoos increased significantly during 1994–2006 in lowland
semi-natural grass, heath and bog but decreased in almost all
other habitat types (Newson
et al. 2009). Due to climate-induced earlier breeding
in recent decades, Dunnock nests
have become less available to Cuckoos and those of Reed
Warblers more so: this may explain a concurrent increase
in the rate of parasitism of Reed Warbler
nests (Douglas et
al. 2010).